1. Field of Invention
This invention relates to a process for forming shaped articles, which can be used as prosthetic devices, which are formed of cross-linked ultrahigh molecular weight polyethylene (xe2x80x9cUHMWPExe2x80x9d). The articles exhibit an improved combination of wear resistance and mechanical properties, rendering them useful as the load bearing component in prosthetic knee joints, prosthetic hip joints and as bearing components for other prosthetic replacement joints for the human body. In particular, the shaped articles formed according to the process are suited for use as the lining of an acetabular cup of a prosthetic hip joint.
2. Description of Related Art
UHMWPE, which possesses an average molecular weight of between about 1 million and about 10 million, is used to make prosthetic devices, such as the ones enumerated above. UHMWPE can be formed into the devices by techniques that include: (1) direct compression molding UHMWPE resin into the final shape of the device; (2) ram extrusion of a UHMWPE powder into cylindrical bar stock that is several inches in diameter, and machining the implant from the bar stock and (3) molding large sheets of UHMWPE and machining the implant from the molded sheet. The molded sheets can be up to 8 inches thick and 8 feet in both width and length.
Artificial hip joints and artificial knee joints must possess high mechanical strength because of the loads these joints will bear over several years. Typically, these joints are constructed of hard metal or ceramic, and are a provided with a relatively softer lining of a UHMWPE at least at the interface of movable part(s). For example, UHMWPE is typically used to construct the lining of an acetabular cup of a prosthetic hip joint.
When used as a load bearing component, UHMWPE typically deteriorates over time due to the continuous wear the component is subjected to. As stated in EP 722 973, xe2x80x9cIn recent years, it has become increasingly apparent that tissue necrosis and interface osteolysis, in response to UHMW polyethylene wear debris, are primary contributors to the long-term loosening failure of prosthetic joints. For example, the process of wear of acetabular cups of UHMW polyethylene in artificial hip joints introduces many microscopic wear particles into the surrounding tissues. The reaction of the body to these particles includes inflammation and deterioration of the tissues, particularly the bone to which the prosthesis is anchored. Eventually, the prosthesis becomes painfully loose and must be revised. It is generally accepted by orthopaedic surgeons and biomaterials scientists that the reaction of tissue to wear debris is the chief cause of long-term failure of such prostheses.xe2x80x9d See page 2 lines 12-19 thereof.
The prior art teaches that UHMWPE xe2x80x9ccan be cross-linked by irradiation with high energy radiation, for example gamma radiation in an inert atmosphere or vacuum. Exposure of UHMWPE to gamma irradiation induces a number of free-radical reactions in the polymer. One of these is cross-linking. The free radicals formed upon irradiation of UHMWPE can also participate in oxidation which reduces the molecular weight of the polymer via chain scission, leading to degradation of physical properties, embrittlement and a significant increase in wear rate. The free radicals are very long-lived (greater than eight years), so that oxidation continues over a very long period of time resulting in as much as a 5-fold increase in the wear rate as a result of oxidation over a period of about 5 years. As such, the wear rate of traditionally irradiated materials is significantly greater than unirradiated materials.xe2x80x9d U.S. Pat. No. 6,017,975 at col. 1 lines 18-35. xe2x80x9cGamma radiation is preferably used in the irradiation step, however electron beam or x-ray radiation may also be used.xe2x80x9d ""975 patent at col. 3 lines 47-49. EP 722 973 provides that xe2x80x9cThe crosslinking can be achieved by various methods known in the art, for example, by irradiation crosslinking of the molten polymer; photocross-linking of the molten polymer; and crosslinking of the polymer with a free radical generating chemical.xe2x80x9d EP 722 973 at page 4 lines 18-21.
However, xe2x80x9cThe free radicals formed upon irradiation of UHMWPE can also participate in oxidation which reduces the molecular weight of the polymer via chain scission, leading to degradation of physical properties, embrittlement and a significant increase in wear rate. The free radicals are very long-lived (greater than eight years), so that oxidation continues over a very long period of time resulting in as much as a 5-fold increase in the wear rate as a result of oxidation over a period of about 5 years. As such, the wear rate of traditionally irradiated materials is significantly greater than unirradiated materials.xe2x80x9d U.S. Pat. No. 6,017,975 at col. 1 lines 26-36.
In addition to teaching irradiation to effect cross-linking in order to improve wear resistance, the prior art teaches that prosthetic devices can be sterilized at the completion of the manufacturing process by irradiating the device. See, e.g., the ""975 patent at col. 1 lines 37-50. However, xe2x80x9csterilization of polyethylene joint components with gamma irradiation followed by storage on a shelf contributes to oxidative degradation. The degradative changes continue with time and occur not only on the surface but also through the bulk of the component. Alterations in the material properties associated with the degradation increase the stresses on the surface of and within the polyethylene total joint replacement components once these components have been implanted and are subject to loading in vivo. Thus, the wear resistance of a polyethylene component that has been stored on a shelf for an extended time may be diminished before the component is implanted.xe2x80x9d The Journal of Bone and Joint Surgery, vol. 76-A, no.7, July 1994 pp. 1052-1055 (see page 1052).
The present invention is a method for forming a shaped article of UHMWPE, such as the components of a prosthetic device, and in particular, the load bearing components thereof. The method comprises the steps of
a) forming, under heat and pressure, ultra high molecular weight polyethylene into a shaped article;
b) cooling the shaped article;
c) cross-linking the ultra high molecular weight polyethylene of the shaped article, preferably by irradiating it, more preferably by irradiating it with gamma radiation;
d) heating the shaped article to a temperature below the melting point of the ultra high molecular weight polyethylene; and
e) sterilizing the shaped article using non-crosslinking sterilization means, such as exposure to ethylene oxide or gas plasma.
Without wishing to be bound by any theory, it is believed that following the irradiation step with the heating step reduces or minimizes the detrimental affects associated with irradiation. The present applicant offers no view as to why this is the case, or what mechanism causes the improvement. The applicant notes that others have offered a hypothesis for what occurs when an irradiation step is followed by heating xe2x80x9cnear or above the melting pointxe2x80x9d. See U.S. Pat. No. 6,017,975 at col. 2 lines 26-30. The applicant expresses no view at this time concerning the correctness of this hypothesis.